The present invention relates to bandgap circuits which generate a substantially temperature insensitive voltage reference signal. Specifically, a bandgap circuit is provided which avoids any metastable operation.
Bandgap circuits are used in bipolar and BiCMOS circuit designs for producing a stable reference voltage. The stable reference voltage is used to control other voltage levels within a chip, and to provide a bias current that is proportional to absolute temperature. The voltage regulation and bias current applications are used extensively in analog circuits such as cellular telephone applications. Bandgap circuits must not only provide the required voltage and current functions, but must be power efficient since the cellular telephone circuits are powered by batteries. The bandgap circuit is integral to the operation of the circuit, and its reliability is essential to avoid catastrophic circuit failure.
The bandgap circuits are known to have three operating states. The first operating state provides a normal operation which produces the required regulated voltage, or bias current. The second state is a zero current state, which means that the circuit is not operable at all. The third operating state is the metastable state representing a circuit failure. One of the more common problems with bandgap circuits is the failure to enter the normal operational state from the zero state. If the bandgap circuit enters the metastable state, the output voltage does not attain a final reference value, and the circuit may remain in the metastable state, with the result that the entire cellular telephone circuit may fail.
The solution to the problem is to provide additional start-up circuitry which forces the bandgap circuit into its normal operational state. However, additional start-up circuitry adds overhead to the power budget for the circuit device placing additional burden upon the battery power supply.
The present invention provides a start-up circuit which unconditionally places a bandgap circuit in its normal stable operational state independent of manufacturing process variations, temperature variations and power voltage supply variations. The pulse start-up circuit does not interfere with normal bandgap operations, and draws no additional current from the power supply once the bandgap circuit is in the normal, stable operational mode.
In accordance with the invention, a start pulse circuit generates a pulse when the power supply voltage is applied to the bandgap circuit. The pulse is applied through a transistor to the regenerative bandgap reference circuit, and forces the output voltage of the regenerative bandgap reference circuit to a voltage higher than the normal output voltage. At the end of the pulse, the regenerative bandgap reference circuit output voltage decreases to a stable normal voltage reference value, avoiding the metastable state.
In accordance with one embodiment of the invention, a momentary start pulse is produced from a logic gate and delay circuit. The enable voltage for the bandgap reference circuit is applied to the delay circuit and a second input of the logic gate. A pulse is formed from the logic gate which is used to drive the regenerative bandgap reference circuit into an overvoltage output state. Once the pulse has ended, the delay circuit and logic gate are effectively decoupled from the bandgap circuit and no additional power burden occurs on the battery power supply.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.